In applications, which are situated on remote places, tuning of circuits could be both expensive and difficult. This can of course also be true for applications located at installations which for other reasons are difficult to reach, such as high up in mast tops or buried in the ground. Nevertheless, setting and tuning of working variables in such circuits could be of vital importance for correct operation, to prevent damaging expensive equipment and saving costs, such as supply power.
One example of such an application could be to set, or tune, the operation point in a power amplifier transistor in a transmitter for a telecommunication application.
In this application the circuit should, by itself, after power up, tune in and hold the correct operation point for the power amplifier transistors in the transmitter. As always, high demands regarding for instance reliability is put on telecommunication application and since the tuning and setting of the operating point for the power transistor is vital for the correct operation of the transistor it is highly important that a secure and reliable tuning and setting is performed.
In prior art applications this could be performed by placing a comparator having two inputs, a first for a reference voltage and a second for a variable voltage, which will vary with the value to be set, in a closed loop comprising a memory. Increasing the value to be set increases the variable voltage until the variable voltage is greater than the reference voltage at which time the comparator changes value. When the comparator changes value the value of the counter is stored in a memory and is used as the value for, for instance the operating point for a power amplifier transistor.
This arrangement has the drawback that if a relatively fast scheme is applied, the counter will count past the optimal value before the comparator changes value. This could partly be due to delays introduced in the system since the value to be set, or tuned, is used in the system to be controlled, which in turn supplies the variable voltage to the comparator. If a slower scheme is applied, tuning will naturally take longer time, which is a major drawback, and nevertheless the delays in the system to be controlled will still affect the results and is oftentimes uncertain.
Even more important is the fact that transients may cause the comparator to change value. Transients may be large compared to the measured value and may cause the comparator to indicate that the variable voltage has passed the reference voltage and thus that the optimal value has been reached, long before the system has tuned in. Noise may also contribute to erroneous changes of the comparator and thus false indications of optimal settings.
Other prior art systems could for instance apply complex, expensive and large microprocessors to achieve a perfect tuning. This solution has the obvious drawbacks of being complex, expensive and taking up large space.